The present invention relates to a torque sensing unit employed in a power steering mechanism of a car, etc. and, more particularly, a torque sensing unit for sensing a torque in a non-contact manner when an external force is applied to a rotation shaft and a method of manufacturing the same.
In the prior art, the power steering mechanism in which, when the driver steers the car by turning the steering wheel, the rotating force is applied from the motor to the steering mechanism in response to the torque applied to the steering wheel so as to assist the steering force mechanism has been employed.
In the power steering mechanism, in order to decide a power assisting amount, the torque applied to the steering wheel must be sensed. As the sensing unit, there is the torque sensing unit disclosed in the Unexamined Japanese Patent Application Publication No. Hei 6-174569, for example. The structure of this sensing unit will be explained with reference to FIG. 4.
In FIG. 4, the steering wheel (not shown) is attached to a first shaft 1, and the pinion gear (not shown) of the steering mechanism is attached to a second shaft 2. Then, a torsion bar 3 is arranged on center shafts of the first shaft 1 and the second shaft 2 to connect elastically two shafts in the twisting direction. The first shaft 1 is supported rotatably to a cylindrical case 4, which is fitted to the body of the car, etc., via a bearing 14a. A first sleeve 14a formed of non-magnetic material is secured to the first shaft 1. A first cylindrical magnetic element 11 and a second cylindrical magnetic element 12, both are formed of soft magnetic material, and are secured on an outer periphery of the first sleeve 14a at a predetermined interval.
The right end edge of the first magnetic element 11 is formed as a flat plane perpendicular to the shaft center of the first shaft 1, and rectangular teeth portions 11a are formed on the left end edge at an equal pitch along the peripheral direction. The right end edge of the second magnetic element 12, which opposes to the first magnetic element 11, is formed as a flat plane perpendicular to the shaft center of the first shaft 1, and rectangular teeth portions 12a are formed on the left end edge at an equal pitch along the peripheral direction. A teeth width dimension of the teeth portion 12a is set substantially equal to a width dimension of a notched portion of the teeth portion 12a. 
A second sleeve 14b formed of non-magnetic material is secured to the second shaft 2, and a third cylindrical magnetic element 13 formed of non-magnetic material is secured to an outer periphery of the second sleeve 14b. A plurality of teeth portions 13a that have the same width, the same shape, and the same pitch as those of the teeth portion formed on the second magnetic element 12 are formed on the right end edge of the third magnetic element 13.
Yokes 22a, 22b, which are formed to have a cup-shaped sectional shape having inner flanges and formed of soft magnetic material are secured onto the inside of the case 4. The yoke 22a has a length to extend the first magnetic element 11 and the second magnetic element 12 such that the center portion in the axial direction can be arranged at a position opposing to the first magnetic element 11 and the second magnetic element 12. Also, the yoke 22b has a length to extend the second magnetic element 12 and the third magnetic element 13 such that the center portion in the axial direction can be arranged at a position opposing to the second magnetic element 12 and the third magnetic element 13.
A first coil 21a for temperature compensation and a second coil 21b for torque sense are wound on the yokes 22a, 22b along the peripheral direction respectively. Then, when the first coil 21a and the second coil 21b are connected to an oscillator (not shown), the yoke 22a together with the first magnetic element 11 and the second magnetic element 12 constitute a magnetic circuit, while the yoke 22b together with the second magnetic element 12 and the third magnetic element 13 constitute a magnetic circuit.
Next, an operation of the above torque sensing unit will be explained hereunder. When the torque is applied to the second shaft 2 from the steering wheel, twisted deformation is caused in the torsion bar 3, so that a relative angular displacement is generated between the first shaft 1 and the second shaft 2. Then, relative phase difference is generated in the peripheral direction between the second magnetic element 12 and the third magnetic element 13, which are secured to respective shafts via the sleeves 14a, 14b respectively. Therefore, opposing areas between the teeth portions 12a and the teeth portions 13a provided to respective magnetic elements 12, 13 to serve as magnetic paths are changed. Since the second coil 21b constitutes a part of the magnetic circuit which passes the yoke 22b, the second magnetic element 12, and the third magnetic element 13, the magnetic reluctance of this magnetic circuit is changed to then change the inductance when the opposing areas between the teeth portions 12a and the teeth portions 13a acting as the magnetic path is changed. Then, if the AC driving current of the frequency several kHz is supplied to the coil to sense the change in the inductance by the sensing circuit (not shown), the torque applied to the torsion bar 3 can be sensed.
Because the inductance of the second coil 21b is changed by not only the torque but also the temperature, the temperature compensation is needed. Since both the first magnetic element 11 and the second magnetic element 12 are secured to the first shaft 1 via the sleeve 14a, the relative angle between them is not changed by the application of the torque and thus the opposing areas between the teeth portions 11a and the teeth portions 12a are also not changed. Accordingly, the inductance of the first coil 21a which is wound at the middle position between the first magnetic element 11 and the second magnetic element 12 is not changed by the torque. However, since the first coil 21a can change its inductance in response to the change in temperature in the same fashion as the second coil 21b, the output which is not affected by the temperature and is in proportion to only the torque can be derived by sensing difference of the inductance between the first coil 21a and the second coil 21b. 
Next, the method of manufacturing the above torque sensing unit will be explained hereunder. The step of coupling the first shaft 1 and the second shaft 2 with the torsion bar 3 produces easily the angular error. For this reason, in order to fix precisely the rotational phase difference between the magnetic elements at the neutral position, first the first sleeve 14 having the magnetic elements 11, 12 is secured to the first shaft 1. Then, the second sleeve 14b having the third magnetic element 13 is not secured to the second shaft 2 but merely fitted to the second shaft 2. After the first shaft 1 and the second shaft 2 are coupled together by the torsion bar 3, the second sleeve 14b is adjusted such that the rotational phase difference between the first sleeve 14a and the second sleeve 14b is set to a predetermined angle, and then secured to the second shaft 2.
Since the torque sensing unit in the prior art is fabricated as mentioned above, there are such problems that the manufacturing steps become complicated and a higher cost is brought about.
Also, in the variable reluctance type torque sensing unit in the prior art, since the first shaft and the second shaft are made of steel in normal case, the magnetic flux generated in the coils flow into the magnetic elements as well as the above shafts. However, there is such a problem that, since the magnetic characteristic of the shafts has large variation and also the temperature characteristic of the shafts is not good, the precision of the torque sensing unit is degraded if a structure in which a great deal of leakage magnetic flux flows into the shafts is employed.
The present invention has been made to overcome the above-mentioned problem, and it is an object of the present invention to provide a torque sensing unit having high sensing precision, a torque sensing sensor module, and a simple method of manufacturing the same.
The invention set forth in aspect 1 has such an aspect that a method of manufacturing a torque sensing unit which comprises a first shaft and a second shaft rotatably arranged in a coaxial manner to oppose to each other, an elastic member for generating a twisted displacement in response to a torque between the first shaft and the second shaft, a magnetic element rotated in a same phase as the first shaft and magnetic elements rotated in a same phase as the second shaft, and coils for sensing reluctances which are changed in compliance with rotational phase difference between these magnetic elements, whereby the torque acting between the first shaft and the second shaft is sensed, the method comprising the steps of preparing a sensor module in which the magnetic element rotated in the same phase as the first shaft and the magnetic elements rotated in the same phase as the second shaft are temporarily fixed to have a predetermined neutral rotational phase difference previously; and releasing temporary fixing after the first shaft, the second shaft, the elastic member and the sensor module have been assembled.
The invention set forth in aspect 2 has such as aspect that a sensor module constituting a part of a torque sensing unit which comprises a first shaft and a second shaft rotatably arranged in a coaxial manner to oppose to each other, an elastic member for generating a twisted displacement in response to a torque between the first shaft and the second shaft, a magnetic element rotated in a same phase as the first shaft and magnetic elements rotated in a same phase as the second shaft, and coils for sensing reluctances which are changed in compliance with rotational phase difference between these magnetic elements, whereby the torque acting between the first shaft and the second shaft is sensed, wherein the magnetic element rotated in the same phase as the first shaft and the magnetic elements rotated in the same phase as the second shaft are temporarily fixed to have a predetermined rotational phase difference.
The invention set forth in aspect 3 has such an aspect that the sensor module further comprises a plurality of pairs of magnetic elements whose reluctances are changed in compliance with the rotational phase difference between the first shaft and the second shaft, and wherein these are temporarily fixed.
The invention set forth in aspect 4 has such an aspect that the sensor module further comprises a base formed of conductive material to surround the first shaft, the second shaft, and both of them, and wherein the magnetic elements are arranged on an outer side than the base.
The invention set forth in aspect 5 has such an aspect that the sensor module further comprises a base formed of conductive material to surround the first shaft, the second shaft, and both of them; and a first supporting member and a second supporting member which are arranged on an outer side than the base; wherein the first supporting member is fixed to the base and the second supporting member is fixed to the second shaft, and magnetic elements are arranged on surfaces of the first supporting member and the second supporting member.
The invention set forth in aspect 6 has such an aspect that the sensor module further comprises a base formed of conductive material to surround the first shaft, the second shaft, and both of them; and magnetic elements which are arranged on an outer side than the base; wherein a caulking portion whose thickness is about half of other portions or less is provided to a portion of the base, which is not hidden under the magnetic elements.
The invention set forth in aspect 7 has such an aspect that the magnetic elements are formed of soft magnetic amorphous metal.
The invention set forth in aspect 8 has such an aspect that the sensor module further comprises temporary fixing member which has a cylindrical band portion, ribs for limiting axial movement of the first magnetic elements and the second magnetic element or supporting members to which these magnetic elements are secured, and a fastening mechanism for fastening the band.
The invention set forth in aspect 9 has such as aspect that a torque sensing unit which comprises a first shaft and a second shaft rotatably arranged in a coaxial manner to oppose to each other, an elastic member for generating a twisted displacement in response to a torque between the first shaft and the second shaft, a magnetic element rotated in a same phase as the first shaft and magnetic elements rotated in a same phase as the second shaft, and coils for sensing reluctances which are changed in compliance with rotational phase difference between these magnetic elements, whereby the torque acting between the first shaft and the second shaft is sensed, the unit comprising a base formed of conductive material to surround the first shaft, the second shaft, and both of them, and wherein the magnetic elements are arranged on an outer side than the base.